CCD (charge coupled device) imagers are commonly used to capture digital images. The CCD imager is a solid state device which consists of an array of photosensors coupled with a CCD shift register. The photosensors convert incident photons to electrons resulting in a charge buildup in the photosensor which is proportional to the total amount of light incident over a given period of time. At the end of this time period (commonly referred to as the "integration time", the time over which electrons are collected) the charge from each photosensor is transferred to a corresponding location in an analog CCD shift register. The packets of charge now can be shifted sequentially onto a capacitor, generating a voltage which can be measured at the output of the imager. Typically, charge from the previous integration time period is shifted out of the device while charge is being integrated in the photosensors, resulting in a delay of one integration time period between integration and readout.
CCD imagers typically fall into two categories: linear array and area array imagers. In a linear array imager, the pixels (individual photosensor locations are commonly referred to as a "pixels") are arranged in one long row. This type of device is used to capture one line of an image at a time. To capture an entire image, many lines must be captured in succession while either the object or the imager is translated past the other. Sometimes, more than one line of photosensors will be fabricated into the same device. When combined with color filtering, color capture of images is possible. Trilinear (R, G, B) imagers are made this way.
In an area array imager, the photosensors are arranged in a two dimensional pattern, allowing for capture of the entire image during one integration time period. Typically, there is still only a one line shift register, requiring that pixel data be read out one line at a time. For this reason, subsequent discussions will deal with line array imagers only, although the techniques described apply to area imagers as well. Color filtering of photosensors in area arrays is also possible.
Many types of artifacts may be introduced into the image as a result of the CCD imager. Among them is "smear". Smear occurs when photo-generated electrons not captured by the potential well of the photosensor, become distributed in the substrate and are captured by the potential well of the shift register. The result is an extra charge "offset" which is added equally to each pixel shifted out of the device, and is proportional to the total amount of light incident on the entire array of photosensors. Part of the smear is contributed during integration, as smear charge is deposited into the shift register before photosensor charge is transferred, and part of the smear is contributed during readout of the shift register. Since charge transfer is delayed by one integration time period relative to charge integration, the smear for a given integration time period contains components from the current integration time period as well as the previous integration time period.
Smear becomes visible in images when both dark and bright regions of the image are captured during a single line. The light from the bright regions causes smear to be added to all pixels, including those which should be dark.
To correct for smear, the smear offset for a given line must be measured or calculated and then subtracted from each pixel's readout value. A common approach involves the use of "light shield" pixels. A light shield pixel is an extra pixel which is fabricated on the device but is completely masked so as not to receive light. Because light is never incident on the photosensor, any charge which accumulates must be from some undesirable source, such as smear. Therefore, the charge from the light shield pixel can be measured in the same way that charge is measured from all other "active" pixels, and then subtracted from the values of the active pixels.
One disadvantage of this approach is that any measurement of the light shield pixel value will have a significant error associated with it. The reason for this is that CCD imagers are inherently noisy devices. Sources of noise include: (1) Dark Noise, which is charge developed as a result of the thermal excitation of free electrons in the photosensor and shift register; (2) Shot Noise, which is a variation in the random arrival of photons as predicted by Poisson statistics; and (3) Electronics Noise, which can be any noise introduced by the processing and measuring electronics.
Since the smear offset is subtracted from every active pixel value for the entire line, any error in its measurement will be added to each of these pixels as well. Each line will have a different error associated with it. In actual viewed images, this error can easily be as objectionable as the smear artifact which is being removed.
Two techniques are commonly used to reduce the measurement error:
1. The values of several light shield pixels in the line are measured and averaged; and PA1 2. The values of light shield pixels from several lines are measured and averaged. PA1 image receiving means for receiving CCD image data; PA1 means for determining a smear scaling factor for smear estimation, the smear scaling factor being determined by a ratio of smear error per given level of illumination; PA1 smear estimation means for determining a value of smear estimation on a per line basis; PA1 means for adjusting the image data corresponding to smear estimation and dark level correction; and PA1 means for applying the adjusted data to means for gain adjustment. PA1 Ns is the amount of shot noise
By averaging several measurements, the error is reduced. Since the noise sources are often uncorrelated, the error in the average is reduced by: EQU error.sub.average =error.sub.single /sqrt(number of measurements)
Even with method 1, however, it may not be practical to manufacture an imager with enough light shield pixels to adequately reduce the error in the measurement. With method 2, the response of the smear correction is slowed because of the filtering effect of averaging previous lines.
From the foregoing discussion it should be apparent that there remains a need within the art for smear correction techniques that provide for imagers that are practical to manufacture without creating imagers that are unduly slow.